The subject matter of this invention is a method for determining the state of charge and loading capacity of an electrical storage battery.
It is particularly important for the user of storage batteries to have sufficiently accurate methods available for determining the state of charge and also the loading capacity and availability of a storage battery for a particular application. These need to be determined, for example, in the case of electric driving and electric hybrid driving in a motor vehicle, for assessing the starting ability of an internal combustion engine, for the availability of the power supply for components such as the electrically operated or driven brake systems, steering systems and steering aids, stabilizing systems and for other important functions, and also for the drive mechanisms for components for controlling and regulating electrical parts, particularly for load connection and load disconnection and for voltage regulation, and for display units informing the operator of the vehicle about the operating state.
A wide variety of methods are known for measuring the state of charge of storage batteries. In many cases, integrated meters (current meters) are used. These methods have very good levels of success for batteries in the field of application of full charging and discharging with only rare intermediate charging operations. Errors may arise as a result of inaccuracies in measurement and computation, parasitic charge losses in the storage battery, the aging of the accumulator, and extractable battery capacities changing with the level of the charging or discharging current. However, it is not possible to use such integrating methods to provide a reliable statement about the present level of performance readiness (the product of current times voltage) of the battery, since, for example, the change in the internal resistance is a function of the complete or partial cycles which preceded in terms of time interval and current level.
The weak points mentioned for the charge balancing method can be largely overcome by forming mathematical models of the storage battery. Methods using mathematical models (descriptive equations or mathematically representable equivalent circuit diagrams), which are evaluated on electronic computer systems with the stipulation of a set of basic data typical of the battery, or equivalent circuit diagram component data, make it possible to detect the state of charge of the battery operated simultaneously in parallel with the model. The limiting factor in these methods is merely the quality of the basic model, and the quality of the input parameters in terms of their correspondence to the battery under consideration.
In particular, regulated zero balancing of the output voltage calculated using the model with the actual battery voltage under the conditions of identical temperature and identical current throughput has been found to be particularly useful as an evaluation method. This type of evaluation also permits estimation of the aging processes in the storage battery.
Such methods are disclosed, for example, in EP 471698 B1 (=WO 90/13823), according to which the process input variables (voltage, current, temperature) of the storage battery are measured and processed in a computer, where, according to the principle of indirect measurement, a predeterminable model in the form of multiple-parameter functions, which represents the state of charge and the physical variables to be determined and their physical interrelationships, is compared with the measured process input variables, and, in the event of discrepancies, the model parameters are adapted for the next measurement. To determine the state of charge, a multiple-parameter description in the form of a model is used, for example. However, EP ""698 discloses no further details about the actual configuration of the model or of the equivalent circuit diagram.
The publication xe2x80x9cLadezustandsschxc3xa4tzung von Bleibatterien mit Hilfe das Kalman-Filtersxe2x80x9d [Estimating the state of charge in lead-acid batteries using the Kalman filter] by P. Lxc3xcrkens and W. Steffens (ETZ Archive, Volume 8 (1986), No. 7, pages 231-236) discloses a method for determining the state of charge, in which the method of state estimation using the Kalman filter is used. That publication also points to a suitable equivalent circuit diagram for a lead-acid storage battery.
The invention relates to a method for determining the state of charge and loading capacity of an electrical storage battery by measuring current, voltage and temperature and comparing the measured values with the corresponding values for the response of an equivalent circuit diagram of the storage battery, the parameters of the components in the equivalent circuit diagram and the state variables being varied such that the measured values are matched and the state of charge and loading capacity are determined from the adjusted parameters and state variables.